Why is the performance of one loud speaker better than that of another? Each comprises a diaphragm, a moving coil and a magnet. Yet for one there may be extravagant claims, whilst for another little may be said. Prices, too, vary widely and in some instances we see the evidence of good mass production involving costly plant and in others a degree of finish revealing individual construction. Briefly, loud speaker performance is governed by the following three factors :
- The properties of the diaphragm and its surround. Thickness, weight and the nature of the material used primarily control the ability of the loud speaker to produce a reasonably uniform sound output over the working frequency range of 30 to 10 kHz. By a series of tests using diaphragms of varying weight and thickness and composed of different materials combined with the several forms, of surround, a specimen may readily be chosen which will produce the best results. There is conclusive evidence revealed by the practice of the majority of loud speaker manufacturers that the most suitable diaphragm is a seamless one, moulded from paper pulp with the aid of press tools, thickened and stiffened towards the centre and possessing increasing pliability as the surround is reached. Measurements for determining performance must necessarily be carried out with the associated output transformer. If considerations of cost prohibit a generous design for this component, it may have inadequate primary inductance but will possess the merit of low leakage. In consequence, the characteristic of a well preserved upper register with a comparatively poor bass will be imparted to the loud speaker, a condition which may be quite effectively off-set by the adoption of a diaphragm which tends towards the exclusion of the higher frequencies while producing a marked resonance in the region of 120 Hz.
- The diameter of the moving coil, This is not dependent upon the ability of the coil to handle the applied speech watts without overloading, for although some five Watts of sustained alternating current would quickly destroy the usual 1.5 inch coil, such a value is only delivered from the amplifier intermittently and the heating effect is not the same as when five Watts of direct current are dissipated across a fixed resistance. Experience shows that the larger the moving coil in relation to the size of the diaphragm it has to drive, the better will be the performance. This is due to the improved facility of the diaphragm correctly to follow the spontaneous changes met.
- The density of the magnetic flux in which the coil moves. The maximum possible flux must be created necessitating careful choice and examination of the iron used for the magnet. It must be practically carbon-free and without contamination by nickel, cobalt and, particularly, manganese. As iron to this specification cannot easily be cast we see the use of built-up magnets using pressings or forgings, the front plate being a strip of metal in which the flux pours in from two of the sides producing a far from uniform flux distribution in the gap. A heavy cylindrical form of magnet construction would seem to be essential and in the Haynes loud speakers this is cast from pure iron which is subsequently carbon-freed by prolonged heat treatment. In this way a uniform flux density is obtained all round the gap and its value far exceeds that present in any form of permanent magnet.
In choosing a loud speaker for quality reproduction one must look for good transient response, the result of careful choice of diaphragm, the use of a not over-supple surround and a magnet producing the highest possible flux density in all parts of the gap. An overall frequency characteristic, assuming that it can be taken with reasonable accuracy, conveys but part of the story.
The home constructor was quick to appreciate the merits of the moving-coil loud speaker, and it was a Haynes design in I927 which first rendered available the necessary castings from which the early loud speakers were built.
Full details of the Haynes Standard and Senior loud speakers are given in a free 32 page booklet, which includes specifications of Haynes Quality Receivers and Amplifiers in chassis form or as complete radiograms. Sets are demonstrated every Friday at the Enfield Factory, between 7.30 and 9.30 p.m.